Inertial measurement systems are used to determine the attitude, position and velocity of an object. Typically an inertial sensor suite comprises a triad of accelerometers which measures the non-gravitational acceleration vector of an object with respect to the inertial frame, and a triad of gyroscopes which measures the angular velocity vector of an object with respect to the inertial frame. Processing the outputs of the inertial sensors through a set of strapdown navigation algorithms yields the complete kinematic state of the object. State-of-the-art commercially available inertial navigation systems can provide position accuracies on the order of 1 nautical mile per hour position error growth rate.
In some contemporary applications it is desirable to know the position of objects relative to each other, rather than in an absolute sense. Further, the accuracy desired is on the order of a centimeter, rather than a nautical mile.
Two exemplary applications that require very accurate knowledge of relative position of objects include radiation emitter location determination systems, and ultra-tightly coupled GPS-inertial navigation systems. These types of systems include a master inertial sensing unit in communication with at least one remote slave inertial sensing unit that is co-located with an antenna. The instantaneous relative position and relative velocity vectors between the master and slave inertial sensor units are required to satisfy the stringent accuracy requirements placed on these systems. The nominal baseline vector between the master and slave inertial sensor units is known in such systems; however, the slave inertial sensor system and master inertial sensor system are often moving relative to each other due to vibration and flexure of the vehicle, and so the baseline is in reality only approximately known.
In an exemplary application, one of the inertial sensor systems is located on the wing of an aircraft in flight and the other is located on the body of the aircraft. In flight, the aircraft undergoes flexure effects at one or more prominent resonant frequencies that cause the relative sensor positions to oscillate about the end positions of the baseline between the master and slave inertial measurement units. In the case where the inertial measurement unit is close to the wingtip of an aircraft, the amount of sensor offset from the baseline can be greater than a meter. Further, in this exemplary application, an antenna co-located with the inertial measurement unit responds to the same large flexure motion. Consequently, unless the relative position is corrected for the flexure motion, user systems that utilize the signal from the antenna may experience degraded performance.
The need exits to determine large amplitude, high-frequency, relative motions of structural elements at centimeter-level accuracies.